Transformer core construction



July 8, 1969 w HOOVER 3,454,916

TRANSFORMER CORE CONSTRUCTION Filed Oct. 9, 1967 RAD/US I I H I F/G. 2

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WILLIAM GEORGE HOOVER EH11, WWI-d; F/ 6 3 m W ATTORNEYS United States Patent 3,454,916 TRANSFORMER CORE CONSTRUCTION William George Hoover, Los Altos Hills, Calif., assignor to Granger Associates, Palo Alto, Calif., a corporation of California Filed Oct. 9, 1967, Ser. No. 673,825 Int. Cl. Htllf 27/08, 27/28, 27/24 U.S. Cl. 336-60 5 Claims ABSTRACT OF THE DISCLOSURE This invention pertains to a transformer construction of the type employing a toroidal core.

It is a general object of this invention to provide an improved transformer of a type employing a toroidal core.

Heretofore, the cores of transformers of a type employing toroidal cores have been subject to premature deterioration and cracking when subjected to prolonged high power transforming conditions.

It is, therefore, an object of the invention to provide a toroidal core configuration which serves to accommodate as high or even higher power transforming conditions as heretofore while reducing the tendency of the core to deteriorate, crack, etc.

During transforming conditions, a toroidal transformer core will become heated. It has been observed that while the overall temperature of the core material can be raised considerably Without deleterious effects, the creation of coexisting differences in temperature between radially displaced portions of the core can create stresses in the material thereby causing deterioration.

It has been further observed that the inner radius of the core will become heated more readily than the outer radius due to the greater density of the magnetic flux induced in the core thereat during transforming operations.

It is, therefore, another object of the invention to minimize the temperature differential created radially of the core without sacrifice in the flux carrying capability of the core.

It is another object of the invention to provide a core configuration which is readily susceptible to being cooled.

It is a further object of the invention to provide a core configuration whereby portions of the toroidal core mem ber can be readily provided to carry a greater flux density so as to increase the inductance of the core for its size. (An increase in the cores inductance provides improved transformer performance.)

Thus, as provided herein, toroidal shaped and concentrically arranged core members are provided with a modest spacing between inner and outer core members whereby a cooling fluid such as air or insulating oil may be passed therebetween serves to provide the above advantages.

In toroidal core constructions, it has been observed that the degree of power losses at the inner radius largely determines the permeability of the ferrite material chosen for the core. This permeability is, of course, substantially the same for all parts of the core. With the concentric ring construction, it is possible to increase the permeability of the larger radius outer rings in a manner such that the total core heating is not excessive, even though the average permeability of the total core cross-section is considerably 3,454,916 Patented July 8, 1969 greater than is safely possible with a comparable unitary core construction.

The graded-permeability principle permits the use of a smaller magnetic core and/ or fewer turns around the core for a given inductance.

The foregoing and other objects of the invention will be more readily apparent from'the following detailed description of a preferred embodiment when considered in conjunction with the accompanying drawings, in which:

FIGURE 1 is a schematic perspective view showing a transformer core construction, according to the invention;

FIGURE 2 is a graph showing a plot of flux density against radius for a portion of the core construction shown in FIGURE 1 and shown in relation to an enlarged detail of a portion thereof positionally related to the traces on the graph; and

FIGURE 3 is a schematic perspective view showing a core construction according to the invention in place for cooperation with a transformer apparatus, according to the invention.

In general, there has been provided a stacked core construction for a transformer comprising concentrically arranged inner and outer toroidal shaped core members of magnetically permeable material adapted to carry an induced flux generated by windings associated therewith. The core members are disposed in radially spaced concentric relation so as to define a fluid flow path between the outer circumference of the inner core members and the inner circumference of the outer core members.

Preferably, the permeability of the outer core member is greater than the permeability of the inner core member. Further, the radial extent of the inner core member is preferably on the order of one-half the radial extent of the outer core member so as to minimize that portion of the core construction which may be limited by virtue of the relatively short flux path extending circumferentially therearound.

Thus, the inner circumference of each core member is considered to constitute something of a limiting factor in the flux density or flux carrying capability for that core member.

Referring to the drawing, there is schematically shown a stack of core pairs comprising core members 11, 12 arranged concentrically of each other. Core members 12 are spaced radially slightly from the inner circumference of core members 11 so as to form a flow passage 13 extending axially along the core construction.

Windings 1'4, 16 constitute, respectively, the primary and secondary windings of the transformer with which the core construction is associated The location and number of turns of each winding 14, 16 may be conventional and the drawing indicates only the existence of such windings and not their actual location or number as employed in practice.

The number and location of such windings, of course, will be readily apparent to those skilled in the art and need not be further described.

The cross-sectional area of core member 12 radially transversely of the stack is shown in FIGURE 2 and provides a flux transmitting cross-section area 17 on the order of half that of the flux transmitting cross-section area 18 of core member 11.

As mentioned above, the foregoing arrangement permits the permeability of the core member 11 to be considerably greater than that of core member 12 by virtue of the fact that the longer flux path defined by the inner circumference of member 11 serves to accommodate greater flux density than would the shorter circumference defined by the inner radius of member 12.

Thus, it has been found that even though some slight reduction in the flux carrying cross-section may be experienced by provision of the flow passages 13 the benefits to be gained by such reduction are considerably offset by avoiding the flux limiting characteristics referred to above.

Referring to FIGURE 2, the characteristic of each pair of core members =11, 12 is shown. As appearing on the graph of FIGURE 2, it will be apparent that each pair of core members may be considered in its function as a single core means 19 having an inner and outer circumference 21, 22 respectively whereby a winding, such as *14 (FIGURE 1) may be disposed and adapted to be energized and thereby induce a magnetic flux in both portions of core means 19. Where the portion formed by member 18 is of greater permeability than the portion formed by member 17, there will be a substantial increase in the flux density at a disposition such as point 23-which is intermediate the inner and outer circumference 21, 22 of the overall core means. Position 23 is, obviously, substantially displaced radially outwardly relative to the inner circumference 21.

In this manner, the usual decay of flux density ranging from the inner circumference to the outer circumference of a toroidal core construction has been substantially improved. I

As mentioned above, flow passages 13 provide an increased surface area available for cooling members 11, 12 as air or insulative oil is arranged to flow therethrough.

Thus, as shown in FIGURE 3, a transformer assembly 24 is shown schematically, inasmuch as in large part such a construction is well known to those skilled in the art.

Transformer assembly 24 includes, for example, an outer cylindrical housing 26 within which is mounted a cylindrical chimney 27 of insulative material. Chimney 27 is spaced somewhat from the inside wall of housing 26 and is generally open longitudinally of its length to form a flue portion therein. A deposit of insulative oil submerges chimney 27 within housing 26 whereby upon operation of the transformer units 28, heat generated thereby will be dissipated by the oil.

Transformer units 28 are mounted on a divider plate 29 on opposite sides thereof.

Each transformer unit 28 is assembled by attaching a plate 31 at each end of a stack of pairs of core members as shown in FIGURE 1. Thus, in FIGURE 3, core member 11 is indicated as '11 prime (11'). Each end plate 31 includes openings 33 disposed to overlie the flow passages 13 definined between core members 11 and 12'.

Conductors 32 form the windings 14, 16 as referred to in FIGURE 1 and are generally for a high power R.F. transformer of the type shown. Thus, as is generally known, such conductors are formed of a relatively large diameter single rod element of copper or other suitable conductor material.

Finally, in order to make connection, a pair of leads of a type such as the one shown in phantom lines 34 are coupled to the transformer units 28 as in a coaxial cable connection (not shown). The foregoing connections will be, of courss, readily apparent to those skilled in the art, and for this reason further detailed description is considered unnecessary.

In operation, transformer units 28 generate considerable heat when transforming relatively high power. As the heat is increased, flow of the insulative deposit of oil in which the units 28 are submerged is induced. By providing flow passages 13, additional peripheral cooling areas are formed for the transformer unit so as to aid cooling as well as to induce additional circulation of the oil within the flue portion of chimney 27. Thus, it will be readily evident that the surface area available for cooling core members 11, 12 has been substantially increased by providing the flow passages 13 extending longitudinally of the stack.

From the foregoing, it will be readily evident that a transformer core construction has been provided having the foregoing advantages.

An additional advantage, not mentioned above, but of considerable importance is that a construction of the foregoing type serves to reduce the length of the windings 14, 16 so as to result in a significant reduction in the heating of the conductors forming windings 14, 16 whereby higher upper frequency operation is achieved.

What is claimed is:

1. In a high power transformer, a core construction characterized by a stack of aligned core pairs, each pair including first and second toroidal core members of magnetically permeable material adapted to carry an induced flux therein, primary and secondary windings each coupled in common to both said members of said pairs, said first and second core members being disposed in spaced concentric relation adapted to define a fluid flow path therebetween, and means forming a flue enclosing said members and serving to induce fluid flow along said flow path.

2. A transformer core construction as defined in claim 1 wherein the outer ones of said core members have a greater magnetic permeability than the magnetic permeability of the other core members so as to increase the total flux density of the outer core members relative to that of the inner core members.

3. A magnetic core construction according to claim 1 wherein the transverse cross-section radially of said core members provides a cross-section for the inner core members of substantially one-half the area for the outer core members.

4. In a high power transformer a core construction characterized by toroidal shaped core means having inner and outer circumference, said toroidal core means comprising a first toroidal core portion having a prede termined magnetic permeability for carrying a related density of magnetic flux therealong, a second toroidal core portion concentrically of said first portion and radially spaced outward thereof to form a fluid flow path therebetween acting as a flue to induce a circulation of fluid therealong, said second portion having a magnetic permeability serving to carry a related magnetic flux density on the order of at least as great as that of the first named said density, primary and secondary windings disposed in common relative to both said portions of said core means, one of said windings being adapted to be energized to induce a magnetic flux in said core means, said core means being formed to provide a substantial increase in the density of said flux at a disposition intermediate said inner and outer circumferences and substantially displaced radially outwardly relative to said inner circumference.

5. A transformer core construction according to claim 4 wherein the magnetic permeability of the second core portion is substantially greater than that of the first portion.

References Cited UNITED STATES PATENTS 2,619,627 11/1952 Slepian 336-211 2,780,786 2/1957 Johnson 336-212 2,829,338 4/1958 Lord 336-212 XR 3,213,431 10/1965 Kolk et al. 340-174 XR 3,315,087 4/1967 Ingenito 307-88 3,317,742 5/1967 Guerth 340-174 XR LEWIS H. MYERS, Primary Examiner.

T. J. KOZMA, Assistant Examiner.

US. Cl. X.R. 

